Precipitated silica is a well-known white reinforcing filler that is used in vulcanizable elastomer compositions that are used in rubber applications, e.g., tires. It is known that generally in order to obtain optimum reinforcing properties, a filler used in elastomer mixtures should be present in a finely-divided form and distributed homogenously in the elastomer mixture. Many of the early precipitated silica materials used in elastomers had a tendency to agglomerate during incorporation into the vulcanizable elastomer mixture, which limited the level of reinforcement imparted to the elastomer by such silica.
Recently, precipitated silicas have been developed for use in what are known in the industry as “green tires”, which has allowed a reduction in the rolling resistance of such tires compared to earlier silica-reinforced elastomers used in tires. However, the abrasion performance of the “green tire” has remained at approximately the level of a tire reinforced with only carbon black. With the increasing cost of raw materials and environmental pressures, there is a continuing need for further improvements in the rolling resistance of tires while also providing such tires with equal or improved abrasion resistance.
The CTAB surface area of precipitated silicas has been shown to correlate directly with reinforcement-related properties in elastomer mixtures. It is generally accepted that a higher CTAB surface area leads to improved abrasion resistance. However, higher CTAB surface areas have also been shown to increase the hysteretic properties of the cured elastomer mixture, i.e., the mechanic-dynamic loading of the cured elastomer mixture causes in the case of tires higher heat generation—a consequence of which is increased rolling resistance, which leads to poorer fuel efficiency.
It is also suggested that the structure, i.e., pores, formed within the precipitated silica during its preparation can have an impact on performance. Two measurements of this structure are the BET/CTAB surface area ratio of the precipitated silica, and the relative breadth (γ) of the pore size distribution of the precipitated silica. The BET/CTAB quotient is the ratio of the overall precipitated silica surface area including the surface area contained in pores only accessible to smaller molecules, such as nitrogen (BET), to the external surface area (CTAB) that is accessible to the elastomer, e.g., rubber, in which the silica is incorporated. This ratio is typically referred to as a measure of microporosity. A high microporosity value, i.e., a high BET/CTAB quotient number, is a high proportion of internal surface—accessible to the small nitrogen molecule (BET surface area) but not to the elastomer—to the external surface (CTAB) that is accessible to the elastomer. The relative breadth (γ) of pore size distribution is an indication of how broadly the pore sizes are distributed within the precipitated silica particle. The lower the γ value, the narrower is the pore size distribution of the pores within the precipitated silica particle.
Finally, it is also known that the silanol groups on the surface of precipitated silica can impact its performance in elastomer mixtures. The Sears number is a measure that describes the concentration of silanol groups on the precipitated silica. One suggested parameter of precipitated silica is the concentration of silanol groups for a given level of CTAB surface area. The silanol groups on the precipitated silica surface function as potential chemical reaction sites for a coupling reagent, which permits coupling of the silica to the elastomer (rubber) matrix, which can lead to improved reinforcement properties, e.g., improved abrasion resistance. The silanol groups on the silica surface in elastomer mixtures also function as sites for particle-to-particle interactions. An increase in particle-to-particle interactions create increases in hysteretic properties, i.e., the mechanic-dynamic loading of the cured elastomer mixture results in higher heat generation, an example of the consequence being increased rolling resistance for tires, which leads to poorer fuel efficiency.